1. Field of the Invention
This invention is concerned with pull-type blind-riveting assemblies.
2. Statement Of The Prior Art
Blind rivet assemblies may comprise a hollow rivet and a mandrel which includes a head and a cylindrical pulling portion, the mandrel having a pyramidal under-surface which provides radially disposed ridges to cause the tail end of the rivet to split into petals when the mandrel head is pulled into it on setting the rivet. Split-setting blind-riveting assemblies are of two kinds, viz. those in which the rivet barrel has longitudinal lines of weakness where it will split into petals when a mandrel head is pulled into it and those where the rivet barrel has no such lines of weakness but splitting is induced by radially disposed ridges on the under-surface of the mandrel head. Such ridges are usually provided by the edges of a three, four or five sided pyramidal configuration of such surface, the pyramidal configuration extending substantially from the periphery of a mandrel head to a cylindrical portion of the mandrel stem.
It is potentially more economical to manufacture the latter type of assembly because the process is less complicated than that required to provide lines of weakness along the rivet shank. However, it is desirable that the effect of the ridges on the under-surface of the mandrel head is to split the rivet shank evenly into the desired number of petals, i.e. four where the under-surface of a mandrel head is part of a square pyramid, and experience has shown that to ensure such a result requires the rivet shank wall to be of uniform thickness within very close limits of the manufacturing tolerance. Where, for example, the rivet bore is eccentric to a small degree outside such tolerance, the shank may split along three lines but not along, or only a short way along, the fourth. The maintenance of the necessary close manufacturing tolerance for the rivet to ensure uniform splitting has proved costly.
An attempt to meet the foregoing problem has been made in the art where a mandrel for this purpose is characterized in that the under-surface of the mandrel head meets a cylindrical portion of the stem at a plane normal to the axis of the stem with the result that at least at each corner of the pyramidal surface the mandrel head presents a flat shoulder portion normal to the mandrel stem. While such a mandrel led to a substantial improvement in the reliability of rivet setting, the problem of irregular setting still remained in an unacceptably high proportion of rivet settings.